Methods for filling holes in printed wiring boards

ABSTRACT

The invention comprises methods for filling holes in printed wiring boards and printed wiring boards produced by these methods. The methods involve plating metal conductors inside the holes of the printed wiring boards while protecting the conducting surfaces of the printed wiring boards from being plated using photoresist film. The side surfaces of a printed wiring board are covered with photoresist. The photoresist is exposed to developing light, except the photoresist covering the holes on one side of the board is masked to prevent exposure of the holes to the developing light. The undeveloped photoresist covering the holes is removed. The board is subjected to a plating process, which deposits conductive materials in the holes, but the photoresist on the conducting surfaces of the board prevents conductive materials to be plated on the surfaces of the board.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. Provisional Application No. 60/443,168, filed Jan. 29, 2003, the disclosure of which is expressly incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

Printed wiring boards (PWBs) are used as platforms for most electronic circuits. They serve as bases for electronic components such as integrated circuit chips, resistors, capacitors, and other familiar circuit elements. PWBs provide the actual physical base for mounting these electronic components, as well as the electrical connections required between the components. PWBs typically consist of a non-conductive dielectric layer, usually epoxy resin coated fiberglass, and a conductive layer, usually copper or some other metal.

To prepare a PWB, the conductive layer is subjected to a set of processes by which much of the conducting metal is removed from the non-conductive substrate, leaving behind a pattern of conductor that provides the proper electrical connections between the electronic components that are ultimately mounted to the PWB (i.e., the circuit pattern). In a photolithography process, the metal layer of the PWB is coated with a light-sensitive photoresist material. A mask is used to create a pattern in the photoresist layer by exposing only those parts of the conducting layer that will comprise the circuit pattern. The PWB is then subjected to ultraviolet developing light. The photoresist material that is on the exposed parts of the PWB, those parts not obscured by the mask, polymerizes on exposure to the ultraviolet light. Next, the mask is removed and the unpolymerized photoresist material is washed off, leaving exposed conductive layer in those parts blocked by the mask, but leaving a layer of polymerized photoresist material over the conductive layer of those parts that were not blocked by the mask. Finally, the PWB is etched in a process wherein the PWB is exposed to an etchant (e.g., cupric chloride, ammoniacal, or sulfuric peroxide). The etchant removes the exposed conductive layer, but leaves the parts of the conductive layer that are protected by the polymerized photoresist.

Modern PWBs often have rather dense circuitry. To accommodate the need for high density circuits, manufacturers have assembled layers of PWBs into multiple layer PWBs, typically with unique circuitry in each layer. These assemblies are typically comprised of alternating layers of PWBs and non-conducting dielectric layers such as epoxy resin coated fiberglass. Epoxy resin is typically the medium used to bind the layers into a single unit.

Multiple layer PWBs require electrical connections not only within each PWB, which is accomplished by photoresist etching as described above, but they also require electrical connections between layers in order to interconnect circuitry on different layers. For example, in a two-layered PWB, with two conductive layers separated by a layer of dielectric, the most efficient use of the PWB area is to allow circuits to span across the two conducting layers. This is typically achieved by drilling holes in the predetermined appropriate places on the multilayered PWB, then depositing a metal on the inside of the holes. The drilling step has become quite sophisticated, and often involves drilling extremely small holes on the order of a hundredth of an inch or smaller. After the drilled hole has been appropriately cleaned by removal of any material at the mouth of the hole, the hole is subjected to a process that makes it conductive. This may involve a non-electrolytic deposition of metal on the inside of the hole, followed by electroplating to the desired thickness, although many other alternative methods of depositing conductor in holes are also used.

When the holes in multi-layered PWBs have been rendered conductive, the deposited metal only coats the surface of the holes, thus effectively creating a hollow cylinder of conductive material. If the number of holes in a PWB is great enough, they can cause the PWB to become quite fragile even after the metal deposit. To reinforce the PWB, the holes may be filled with epoxy resin. Epoxy resin filling was commonly used for relatively large holes, i.e., holes with diameters of greater than about 0.020 inches, with relatively small aspect ratios (the ratio between the length of the hole to its diameter) where small aspect ratios are less than about 7. However, the modern trend has been towards smaller holes with greater aspect ratios, and these smaller holes have become almost impossibly difficult to fill with resin.

There is an ever present need in this art for better methods for filling holes in multi-layered PWBs. There is a particular need for methods for filling small holes with large aspect ratios in multi-layered PWBs.

SUMMARY OF THE INVENTION

The present invention provides methods for filling holes in multi-layered PWBs. The method is particularly useful for filling small holes (less than 0.02 inches in diameter) with large aspect ratios (greater than about 7) in multi-layered PWBs.

In one aspect, the invention includes a method for filling at least one hole in a PWB. A photoresist material may be applied to both sides of a multi-layered PWB and the PWB may be exposed to developing light, but the developing light may be blocked and prevented from developing the photoresist material that covers the openings of the holes on at least one side of the PWB. In one embodiment, the photoresist material covering the openings of the holes on both sides of the PWB may be blocked. The PWB may be washed to remove the unexposed photoresist material covering the openings of the holes.

The inner surfaces of the holes may be plated with a conductor. The photoresist material on the surfaces of the PWB may substantially prevent the surfaces of the PWB from being plated with the conductor. In one embodiment of the invention, the conductor plated on the inner surfaces of the holes is copper. In another embodiment, the plating step is performed by electroplating. This method may be used to completely fill holes with conductor, it may be used to substantially fill the hole with conductor (that is when the conductor deposit occupies more volume of the hole than empty space), or it may be used simply to plate the inner surface of the hole with conductor.

This method may be used to fill any hole, but is particularly useful where other hole filling technologies fail, that is where the diameter of the hole to be filled is less than about 0.02 inches, particularly where the diameter of the hole to be filled is less than about 0.01 inches, or where the aspect ratio of the hole to be filled is greater than about 7, particularly where the diameter of the hole to be filled is greater than about 10.

In another aspect, the invention includes PWBs with holes that are filled by the method described above. In particular, the invention includes a PWB with a filled hole, wherein the hole was drilled by any means known to those skilled in the art, including but not limited to mechanical drilling, punching, and laser drilling. Both sides of the PWB may be covered with photoresist, and on one side, those parts of the photoresist covering the holes may be blocked from exposure to developing light and the remainder of the photoresist may be exposed to developing light. In an alternative embodiment, those parts of the photoresist covering the holes may be blocked from exposure to developing light on both sides of the PWB. The PWB may be washed to remove the unexposed photoresist material covering the openings of the holes. The inner surface of the holes may be plated with a conductor without exposing the surfaces of the PWB to be plated. The holes in the PWBs of the invention may be completely filled, substantially filled, or plated using the method of the invention.

The invention includes PWBs with any size holes that have been filled using this method, but the invention particularly includes PWBs with holes less than 0.02 inches in diameter, even more particularly with holes less than 0.01 inches in diameter, and holes with aspect ratios greater than 7, and more particularly with aspect ratios greater than 10.

In yet another aspect, the invention includes an apparatus for the manufacture of PWBs with holes filled by the method described above. The apparatus may include a means for applying photoresist material to a PWB and a means for developing the photoresist on particular parts of the PWB not covering holes in the PWB to be plated. The apparatus may further include a means for washing the board to remove the unexposed photoresist material covering the holes to be plated and a means for plating the hole with a conductor.

DESCRIPTION OF THE DRAWING

FIG. 1A shows a printed wiring board of an embodiment of the invention.

FIG. 1B shows a making a hole in the printed wiring board of FIG. 1A.

FIG. 1C shows plating on the sides of the hole in FIG. 1B.

FIG. 1D shows adding photoresist to the top and bottom of the printed wiring board in FIG. 1C.

FIG. 1E shows removing photoresist adjacent the hole in FIG. 1D.

FIG. 1F shows the hole of FIG. 1E filled with conductor.

FIG. 1G shows the printed wiring board of FIG. 1F with the photoresist removed.

DETAILED DESCRIPTION

The invention includes methods for filling holes in PWBs, PWBs manufactured by these methods, and a manufacturing apparatus for manufacturing PWBs by these methods.

FIG. 1 shows the results on a two-layer PWB following each of the steps of the method. FIG. 1A shows a two-layer PWB with a first copper layer 101, a dielectric layer 102, and a second copper layer 103. While copper is being used for purposes of describing this embodiment, in alternative embodiments any conductor may be used, including, but not limited to, gold, silver, and zinc.

FIG. 1B shows the two-layer PWB with a hole 104 drilled through it. The hole 104 may be drilled using any method known to those in the art for producing holes in PWBs including, but not limited to, mechanical drilling, punching, and laser drilling.

FIG. 1C shows the two-layer PWB after the inner surface of the hole has been plated with copper 105. This plating step may be accomplished using any method known to those skilled in the art for depositing a conductor in holes. In one embodiment, this plating step is accomplished by connecting both surfaces of the PWB to an electrode and electroplating the inner surface of the hole.

FIG. 1D shows the two-layer PWB after a layer of photoresist film has been applied to the top surface 106 and the bottom surface 107 of the PWB. The photoresist film preferably covers the entire top and bottom surfaces of the PWB in order to protect these surfaces from being plated in subsequent steps.

FIG. 1E shows the two-layer PWB after the photoresist film has been exposed to developing light, and the undeveloped photoresist film over the hole in the PWB has been removed, leaving a hole in the photoresist film 108 directly over the opening of the hole 104 in the PWB. In one embodiment, the hole in the photoresist film 108 is only on the top side of the PWB 106. In alternative embodiments, there may additionally be a hole in the photoresist film on the bottom side of the PWB 107.

FIG. 1F shows the two-layer PWB after the hole has been filled with copper 109. The photoresist film on the top 106 and bottom 107 of the PWB prevent additional copper from being deposited on the top and bottom surfaces of the PWB, although a nodule of copper 109 may rise above the top surface of the PWB.

FIG. 1G show the completed two-layer PWB after the hole has been filled with copper, the top surface has been planarized, and the photoresist film has been removed from the top and bottom surfaces of the PWB. 

1. A method for filling at least one hole in a printed wiring board, the method comprising the steps of: providing a printed wiring board with a hole; plating an inner surface of the hole in the board with a first conductor; applying photoresist material to a first side and a second side of the board; exposing a portion of the photoresist material on the first side of the board to developing light, wherein the portion of the photoresist material on the first side of the board that is not exposed to developing light covers an opening of the hole on the first side of the board; exposing the photoresist material on the second side of the board to developing light; washing the board to remove unexposed photoresist material, wherein the unexposed photoresist material that covers the opening of the hole on the first side of the board is removed; and filling the hole in the board with a second conductor; wherein the photoresist material substantially prevents the second conductor from contacting the first and second sides of the board during said filling.
 2. The method of claim 1, wherein the first conductor is copper.
 3. The method of claim 1, wherein the plating step is electroplating.
 4. The method of claim 1, wherein the diameter of the hole is less than about 0.02 inches.
 5. The method of claim 4, wherein the diameter of the hole is less than about 0.01 inches.
 6. The method of claim 1, wherein the diameter of the hole is less than about 0.02 inches, and an aspect ratio of the hole is greater than about
 7. 7. The method of claim 1, wherein the aspect ratio of the hole is greater than about
 7. 8. The method of claim 7, wherein the aspect ratio of the hole is greater than about
 10. 9. The method of claim 1, wherein the hole is substantially filled with the second conductor. 